Debuted: March 1999
Origin: U.S.
Access method: E-mail attachment
Identifying trait: Appeared to be legitimate e-mail, thanks to subject line reading "important message from..." followed by a real user's name.
What it did: Replicated itself, sending copies to others in user's address book.
Scope: Unknown
Cost: At least $80 million*
Postscript: Followed by multiple variants. Virus creator sentenced to 20 months in prison and $5,000 fine.
* U.S. Department of Justice estimate